This invention relates to the volumetric measurement of fluid flow and, more particularly, to means for improving the long-term accuracy of such measurements when carried out by determining the differential pressure across an in-stream precision orifice and solving a formula in which the differential pressure is a variable factor.
Accurate volumetric measurements of gas flow, such as those made in the commercial production and delivery of natural gas, are often carried out by calculations based upon the pressure differential observed across a standard orifice disposed in the gas flow stream. According to the standards established by the American Gas Association, as set forth in Gas Measurement Committee Report No. 3 (Orifice Metering of Natural Gas), the gas flow rate is determined by using the formula: ##EQU1## in which Q.sub.h =the quantity of flow at base conditions in cubic feet per hour
C'=the orifice flow constant PA1 h.sub.w =the differential pressure in inches of water at 60.degree. F. PA1 p.sub.f =the absolute static pressure in pounds per square inch PA1 F.sub.b =basic orifice factor PA1 F.sub.r =Reynold's number factor PA1 Y=expansion factor PA1 F.sub.pb =pressure base factor PA1 F.sub.tb =temperature base factor PA1 F.sub.tf =flowing temperature factor PA1 F.sub.g =specific gravity factor PA1 F.sub.pv =supercompressibility factor PA1 F.sub.m =manometer factor (for mercury type meters only) PA1 F.sub.a =orifice thermal expansion factor PA1 F.sub.1 =gage location factor
The orifice flow constant is defined as the rate of flow in cubic feet per hour, at base conditions, when the extension .sqroot.h.sub.w p.sub.f equals one. It is calculated by the following equation: EQU C'=F.sub.b F.sub.r Y F.sub.pb F.sub.tb F.sub.tf F.sub.g F.sub.pv F.sub.m F.sub.a F.sub.e
where
The values of the several factors are readily ascertainable as discussed in detail in the above-mentioned American Gas Association publication and in other standard references in the field.
It will be manifest that the orifice itself must adhere quite closely to predetermined dimensions and standards of finish in order to obtain and maintain the desired degree of accuracy in calculating the gas volumetric flow. By way of example, a number of specific criteria for the orifice are set forth in the above-referenced American Gas Association publication.
Thus, those skilled in the art will appreciate that the quality of the volumetric measurement will be a strong function of the condition of the orifice plate and in particular of the edges of the orifice itself. These edges, however, are subject to wear by erosion primarily caused by particulate matter in the flowing stream. The amount of wear on the orifice plate depends primarily on (1) the amount of particulate material in the streams; (2) the velocity of the particles; and, (3) the abrasive characteristics of the particles.
Since it is highly desirable, particularly in view of the remarkable increase in the cost of natural gas, that the measuring apparatus operate with sustained accuracy, it will be apparent to those skilled in the art that it would be highly desirable to provide means for compensating for the effects of wear to the orifice plate due to erosion.
It is therefore a broad object of my invention to provide improved means for effecting volumetric measurements of gas flowing through a conduit.
It is another object of my invention to provide means for improving the long-range accuracy of such volumetric measurements.
In a more specific aspect, it is an object of my invention to provide, in an orifice plate apparatus for measuring the rate of gas flow in a conduit, means for indirectly measuring and compensating for orifice plate wear in order to achieve long-range precision of the measuring system.